Annoying pop-up ads aside, this article is quite helpful on this subject -
I quite like the chair analogy from HG Wells:
"Take the word chair. When one says chair, one thinks vaguely of an average chair. But collect individual instances, think of armchairs and reading chairs, and dining-room chairs and kitchen chairs, chairs that pass into benches, chairs that cross the boundary and become settees, dentists’ chairs, thrones, opera stalls, seats of all sorts, those miraculous fungoid growths that cumber the floor of the Arts and Crafts Exhibition, and you will perceive what a lax bundle in fact is this simple straightforward term. In co-operation with an intelligent joiner I would undertake to defeat any definition of chair or chairishness that you gave me."